Effect of brain shift on the creation of functional atlases for deep brain stimulation surgery

Abstract

Purpose: In the recent past many groups have tried to build functional atlases of the deep brain using intra-operatively acquired information such as stimulation responses or micro-electrode recordings. An underlying assumption in building such atlases is that anatomical structures do not move between pre-operative imaging and intra-operative recording. In this study, we present evidences that this assumption is not valid. We quantify the effect of brain shift between pre-operative imaging and intra-operative recording on the creation of functional atlases using intra-operative somatotopy recordings and stimulation response data.

Methods: A total of 73 somatotopy points from 24 bilateral subthalamic nucleus (STN) implantations and 52 eye deviation stimulation response points from 17 bilateral STN implantations were used. These points were spatially normalized on a magnetic resonance imaging (MRI) atlas using a fully automatic non-rigid registration algorithm. Each implantation was categorized as having low, medium or large brain shift based on the amount of pneumocephalus visible on post-operative CT. The locations of somatotopy clusters and stimulation maps were analyzed for each category.

Results: The centroid of the large brain shift cluster of the somatotopy data (posterior, lateral, inferior: 3.06, 11.27, 5.36 mm) was found posterior, medial and inferior to that of the medium cluster (2.90, 13.57, 4.53 mm) which was posterior, medial and inferior to that of the low shift cluster (1.94, 13.92, 3.20 mm). The coordinates are referenced with respect to the mid-commissural point. Euclidean distances between the centroids were 1.68, 2.44 and 3.59 mm, respectively for low-medium, medium-large and low-large shift clusters. We found similar trends for the positions of the stimulation maps. The Euclidian distance between the highest probability locations on the low and medium-large shift maps was 4.06 mm.

Conclusion: The effect of brain shift in deep brain stimulation (DBS) surgery has been demonstrated using intra-operative somatotopy recordings as well as stimulation response data. The results not only indicate that considerable brain shift happens before micro-electrode recordings in DBS but also that brain shift affects the creation of accurate functional atlases. Therefore, care must be taken when building and using such atlases of intra-operative data and also when using intra-operative data to validate anatomical atlases.

Fig. 1

Frontal portion of a sagittal slice (after a 90° rotation for easy visualization) on post-operative CT for patients in a low, b medium, c large shift groups showing air pocket. Width of the air pocket is measured from the inner table of the calvarium at the lead level as shown. Sample measurements in mm are shown. d Typical head orientation in the scanner with the patient lying supine. The dashed horizontal line in a, b and c is a vector passing through the implant in the direction of gravity

Fig. 2

Centroids of somatotopy clusters of low (red), medium (blue) and large (green) brain shift patients overlaid on the atlas MRI. a Sagittal view showing a lateral ventricle and b coronal view showing the interpeduncular cistern for reference

Fig. 3

Axial and coronal slices of the eye deviation maps containing the high probability points for a low brain shift, b medium-large brain shift groups overlaid on the atlas MRI, c color scale